Many vessels in human and animals transport fluids from one body location to another. In some vessels, natural valves are positioned along the length of the vessel to permit fluid flow in a substantially unidirectional manner along the length of the vessel. These natural valves are particularly important in the venous system of the lower extremities to prevent blood from pooling in the lower legs and feet during situations, such as standing or sitting, when the weight of the column of blood in the vein can act to prevent positive blood flow toward the heart. A condition, commonly known as “chronic venous insufficiency”, is primarily found in individuals where gradual dilation of the veins, thrombotic events, or other conditions prevent the leaflets of the native valves from closing properly. This leads to significant leakage of retrograde flow such that the valve is considered “incompetent.” Chronic venous insufficiency is a potentially serious condition in which the symptoms can progress from painful edema and unsightly spider or varicose veins to skin ulcerations. Elevation of the feet and compression stockings can relieve symptoms, but do not treat the underlying disease. Untreated, the disease can impact the ability of individuals to maintain their normal lifestyle. The mechanism involved in the development of chronic venous disease are reviewed in John J. Bergan et al., “Chronic Venous Disease”, N. Engl. J. Med. 2006; 355: 488-98.
A number of surgical procedures have been employed to treat venous valve insufficiency by improving or replacing the native valve. Such procedures include reconstruction of damaged valves with in-situ or exogenous tissue. These efforts have met with limited success and have not been widely adopted as methods of treating chronic venous insufficiency. More recently, efforts have been directed towards finding a suitable self-expanding or otherwise-expandable artificial valve prostheses that can be placed using minimally invasive techniques, rather than requiring open surgery and its obvious disadvantages. Thus far, use of prosthetic venous valves has remained experimental only.
Prosthetic valves have been developed that use a support frame. Frequently, a graft member is attached to the support frame and provides a valve function to the device. For example, the graft member can be in the form of a leaflet that is attached to a frame and movable between first and second positions. In a first position, the valve is open and allows fluid flow to proceed through a vessel in a first direction. In a second position the valve is closed to restrict fluid flow in a second, opposite direction. Examples of such prosthetic valves are described in U.S. Pat. No. 6,508,833, filed Mar. 21, 2001, and U.S. Publication No. 2004/0186558, published Sep. 23, 2004. Another example of a prosthetic valve assembly, including a valve seat and a movable valve composed of a flexible member, is provided by U.S. Pat. No. 5,413,599, filed Dec. 13, 1999. Other known prosthetic valves are attached directly to the vessel wall and do not include a support frame. Examples of such frameless valves are described in U.S. Publication No. 20060265053, published Nov. 23, 2006.
One problem limiting the use of such devices is the potential for thrombus formation, particularly within the valve pockets and other areas where low fluid flow rates can result in pooling and stagnation of blood. Another challenge is the positioning of a device within a body vessel, such as a vein, that dynamically changes its diameter by as such as 50%, due mostly to hydrostatic pressure variations within the fluid within the vessel. For example, such changes within the venous system can occur as a result of everyday activities, such as standing or sitting, or an improvement in patient and/or venous return during treatment.
Other implantable devices, known as self-expanding filters, have been used for temporary or permanent placement within vessels. These devices include a number of anchoring legs diverging from a central hub positioned at one end of the device. For example, such devices have been placed within the vena cava to prevent thrombi or emboli from reaching a patient's lungs and causing a pulmonary embolization. Examples of such devices are described in U.S. Pat. No. 5,324,304, issued Jun. 28, 1994 and U.S. Publication No. 2002/0193828, published Dec. 19, 2002.